1. Field if the Invention
The present invention relates to a honeycomb structure, an exhaust gas purifying apparatus, and a method for producing a honeycomb structure.
2. Discussion of the Background
In recent years, particulate matter (hereinafter, also referred to as “PM”) such as soot discharged from internal combustion engines of vehicles such as buses and trucks, construction machines and the like has raised serious problems as contaminants harmful to the environment and the human body. For this reason, various honeycomb filters, which use a honeycomb structure including a porous ceramic such as cordierite and silicon carbide, have been proposed as filters that capture PM in exhaust gases and purify the exhaust gases.
Further, there has been known a honeycomb structure functions as a catalyst supporting carrier, by supporting a catalyst thereon, for converting CO (carbon monoxide), HC (hydrocarbon), NOx (nitrogen oxides) and the like in exhaust gases.
Furthermore, there has also been known a catalyzed DPF (diesel particulate filter), which is a honeycomb filter supporting a catalyst thereon.
Currently, the exhaust gases from a diesel car are purified (converted) by a combined use of DOC (diesel oxidation catalyst) including a catalyst supporting carrier for converting CO and HC and DPF for capturing PM.
However, since the exhaust gas regulations are expected to become harder in the future, conversion of NOx using NSC (NOx storage catalyst), LNC (lean NOx catalyst), SCR (selective catalytic reduction) or the like is desired.
As an exhaust gas purifying (conversion) system for diesel cars satisfying harder exhaust gas regulations, exhaust gas purifying (conversion) systems using the DOC, DPF, and one of NSC, LNC and SCR in combination have been proposed.
More specifically, a SCR system (inlet-side DOC+SCR catalyst+DPF+outlet-side DOC), a LNC system (DOC+DPF+LNC), and a NSC system (DOC+NSC+DPF) and the like have been proposed.
In order to purify (convert) the exhaust gases by using the above exhaust gas purifying (conversion) systems, a plurality of catalyst supporting carriers and filters including the honeycomb structures need to be disposed in parallel.
However, disposing of a plurality of honeycomb structures may cause the problem that a volume of the honeycomb structures necessary for purifying (converting) the exhaust gases is increased.
To solve the above problem, there has been considered to reduce a number of honeycomb structures to be disposed and to reduce the volume of the honeycomb structures necessary for converting a toxic component by using a catalyzed DPF having an excellent purifying (converting) performance equal to or higher than that of the conventional honeycomb structure.
In consideration of the performance desired for the honeycomb structure having a high NOx converting performance, the honeycomb structure needs to support a larger amount of catalyst compared to the conventional honeycomb structure used as the DOC. Accordingly, the honeycomb structure capable of supporting a large amount of catalyst is desired.
More specifically, the honeycomb structure capable of supporting 100 g/L or more of catalyst, more desirably 250 g/L or more of catalyst, is desired.
In order to support a large amount of catalyst on the honeycomb structure, it is necessary to increase a porosity of the honeycomb structure. More specifically, it is considered that the honeycomb structure needs to have the porosity of 60% or more, more desirably 75% or more.
WO07/129430 A1 discloses one example of the honeycomb structure having a high porosity.
WO07/129430 A1 discloses the honeycomb structure having a high porosity, which is integrally molded through extrusion molding of a material mainly including inorganic fibers.
Since the honeycomb structure including the inorganic fibers is not a sintered body of ceramic particles, the honeycomb structure including the inorganic fibers suitably has a high porosity. WO07/129430 A1 describes a honeycomb filter having a honeycomb structure with cells sealed at either one end by a lamination member for an end portion such as a metal plate.
WO07/129430 A1 further discloses that a catalyst may be supported on part of the inorganic fibers constituting the honeycomb structure.
The contents of WO07/129430 A1 are incorporated herein by reference in their entirety.